The present invention relates to temperature distribution analyzers for analyzing the temperature distribution of a target region by supplying a pulse light to an optical fiber and analyzing the response signal returned from the fiber.
Conventionally, devices are known which have an OTDR (Optical Time Domain Reflectometry) device and an optical fiber. The optical fiber is provided so as to extend through a target region, the temperature distribution of which is to be analyzed. The OTDR device provides a light source, a photodetector and an analyzer. An input pulse light is generated by the light source and is input to the input terminal portion of the optical fiber. The input pulse light then propagates through the optical fiber toward the other terminal portion thereof. There is a tendency, however, for a portion of the input pulse light which is propagating along the length of the fiber, and in an orientation substantially parallel to the longitudinal axis thereof, to disperse or scatter at each point along the optical path. This is the so-called "scattering phenomenon". In the optical fiber, two kinds of scattering are known: the Rayleigh-scattering phenomenon and the Raman-scattering phenomenon. These phenomena result in scattered light including Rayleigh-scattered light and Raman-scattered light. A portion of scattered light is reflected back to the input terminal of the optical fiber by the backward scattering of light. This backscattered light includes Raman-backscattered light. The Raman-backscattered light includes Anti-Stokes light, the wavelength of which is shorter than that of the input pulse light by a predetermined length, and Stokes light, the wavelength of which is longer than that of the input pulse light by a predetermined length. The intensities of the Anti-Stokes light and Stokes light are extremely sensitive to the temperature of the optical fiber. For this reason, the Raman-backscattered light is picked up from the input terminal of the optical fiber in order to analyze the temperature distribution of the target region. The Raman-backscattered light is supplied to the photodetector. The photodetector then converts the Raman-backscattered light to an electrical signal. The electrical signal is sampled by predetermined sampling intervals to obtain sample data. These sample data indicate the variation over time of the intensity of the Raman-scattered light since the input pulse light is input to the optical fiber. The analyzer then analyzes the sample data to determine temperatures at the points along the length of the fiber.
The intensity of the Raman-backscattered light as obtained above is 10.sup.-8 of the intensity of the input pulse light, so that it is necessary for the temperature distribution analysis to provide a high-intensity light source which supplies a high-intensity input pulse light to the optical fiber. Semiconductor lasers are not likely to be used as the light source because the intensities of their outputs are low; sufficiently high intensity Raman-backscattered light cannot be obtained. Recently, LD (Laser-Diode) pumped solid state lasers have come into practical use. The LD pumped solid state laser has a configuration similar to that of conventional solid state lasers in which a pumped light source and a solid crystal for laser oscillation are provided. The conventional solid state laser provides a flash lamp as the light source for pumping, whereas the LD pumped solid state laser provides a LD instead of the flash lamp. The LD pumped solid state laser can supply light at high efficiency and can be of reduced size. LD pumped solid state lasers exist which have a Nd-doped YAG (Y.sub.3 Al.sub.5 O.sub.12) or YLF (LiYF.sub.4) as the solid crystal, the oscillation frequency of which is 1.06 .mu.m or 1.32 .mu.m. Generally, the LD pumped solid state laser having a solid crystal, the oscillation frequency of which is 1.32 .mu.m, is used for applications employing a single mode optical fiber.
However, in the case where a pulse light having a wavelength of 1.32 .mu.m is supplied to an optical fiber by a LD pumped solid state laser, the Raman-scattered light having Anti-Stokes light, the wavelength of which is 1.25 .mu.m, and having Stokes light, the wavelength of which is 1.40 .mu.m, is generated in the optical fiber. Conventional optical fibers have cut-off frequencies in the range of 1.25 .mu.m to 1.28 .mu.m. Accordingly, there are cases in which the wavelength of the Anti-Stokes light is shorter than the cut-off wavelength of the optical fiber and the wavelength does not satisfy a condition for the single mode transmission of the Anti-Stokes light. In these cases, the transmission loss for the Anti-Stokes light becomes large. Furthermore, the intensity of the Anti-Stokes light obtained can vary in response to a number of effects such as bending of the optical fiber. Accordingly, it is difficult to analyze the temperature distribution at high precision and to maintain the stability of the analysis.
A method may be considered in which the Stokes light having a wavelength longer than the cut-off wavelength of the optical fiber is used for the analysis. However, the sensitivity of the intensity of the Stokes light is about 1/7 of that of the Anti-Stokes light for the same variation of the temperature of the optical fiber. Accordingly, it is difficult to achieve the analysis at high precision.